In various industrial applications, it is often desirable to accurately control the pressure of a process fluid stream at a certain point in an industrial system. The process fluid can be gas, liquid, or mixture thereof. For example, in a fuel cell system or a fuel cell testing system, it is necessary to operate the fuel cell under a controlled pressure condition. A common technique is to control the pressure of a process fluid stream adjacent the inlet of the fuel cell for that process fluid stream. To this end, a pressure regulator is often utilized to control the pressure at this point.
A known pressure control system comprises an unloading type flexible element pressure regulator and a piloting system. The pressure regulator is disposed in the process fluid stream line and in operation, allows the process fluid to flow through a chamber thereof disposed on one side of the flexible element, e.g. a diaphragm. A pilot controller senses the pressure at the point where the pressure of the process fluid stream is to be accurately controlled, and correspondingly controls the pressure of another chamber on the other side of the flexible element of the pressure regulator, thereby eventually balancing the pressure on both sides of the flexible element. By manually adjusting the preset pressure of the pilot controller, the pressure of the process fluid stream at the desired point can be controlled. Such pressure regulator and the piloting system and the way they are operated are commercially available from, for example, Mooney Controls.
However, it has been found that this type of pressure control system cannot meet the increasingly strict requirement in terms of accuracy and stability, when a system operates at low pressure, e.g. less than 7 psig, partly because that the pressure regulator needs a certain pressure drop across it to activate the pressure balancing mechanism and such pressure drop is often not available when pressure of the process fluid stream is extremely low.
Therefore, there remains a need for a pressure control system that accurately controls the pressure of a process fluid stream at a certain point, under a range of pressure conditions including very low pressure of the process fluid stream.